Richard J. Davidson, Ph.D., is the Vilas Professor of Psychology and Psychiatry at the
University of Wisconsin-Madison.

Individuals vary considerably in how they react to emotionally challenging events, to life's slings and arrows. With today's imaging procedures, we are learning how individual differences in the activation levels in different parts of the brain play an
important role in regulating our responsivity to emotional events.

The prefrontal cortex appears to play a critical role in the uniquely human capacity
to modulate emotions. Individual differences in amygdala function also appear to affect
certain parameters of emotional reactivity or emotional traits.

Scientists have known for more than a century that when patients sustain damage
to one hemisphere of their brain compared to the other, there are often differences in
emotional reactions. Damage to the left side of the brain, which leaves the right side in
control, was more often associated with a negative mood, including symptoms of
uncontrollable crying and other indicators frequently associated with depression.
Damage to the right side of the brain, when the left was spared, was reported to be
associated with a very different, more positive constellation of mood reactions.

In fact, using a variety of techniques, we have found that the left prefrontal cortex
participates with other structures in a circuit that may be important for certain types of
positive emotion. Activation patterns in the right prefrontal cortex, by contrast, are
more associated with certain types of negative affect accompanied by increased
vigilance to threat-related cues, a symptom that often occurs with certain types of
anxiety.

Some of our experiments involve showing people a series of pictures that differ in
their emotional content. These are standard images that have been used by scores of
researchers on tens of thousands of subjects and are known to produce reliable
reactions. Repeatedly showing more negative or more positive images over a period of
time can induce a pronounced change in mood, as measured by changes in the startle
reflex, by measures of facial expression, as well as by changes in the subjects' reports
of emotional experience.

When a mood change is induced, there is an accompanying change in patterns of
regional glucose metabolism in the brain, which can be assessed with PET scanning.
These changes are consistent with observations of patients with damage to the left
or right prefrontal cortex and to other structures with which the prefrontal cortex is
connected. Using functional magnetic resonance imaging (fMRI) to detect increases in
cerebral blood flow, we found bilateral activation of the amygdala when subjects are
shown negative pictures as opposed to neutral pictures, confirming the amygdala's
involvement in negative or stressful emotions.

In another study, we tested ninety individuals, and then segregated them into those
who showed extreme left-sided activation in the prefrontal region, and those who
showed extreme right-sided activation. Then we asked them to select from a list of
positive and negative adjectives words that described their usual disposition--how they
felt most of the time. An individual who is high in positive affect will rate him or
herself as strong, enthusiastic, alert, proud, excited. On the negative side, we have
adjectives such as distressed, scared, and nervous. The subjects who showed the
left-sided activation reported more positive and less negative affect than their
right-activated counterparts.

This pattern of left prefrontal activation appears to be associated with a
constellation of positive attributes, including reduced levels of the stress hormone
cortisol, and reductions in other biological and immune parameters that are associated
with negative affect.

Our new imaging work suggests that the prefrontal cortex--and particularly the left
prefrontal cortex--may be important in inhibiting activity in the amygdala and
dampening response to negative events, and particularly in shutting off the negative
response quickly once it has been activated. Being able to shut off a negative emotion
appears to be a very adaptive feature and something that individuals who show this
pattern of left-sided brain activity do exceedingly well.

There are now good data to suggest that treatment of depressed patients with
antidepressant medication can change this parameter of prefrontal activation, indicating
there is some plasticity in the system. I suspect that objectively verified psychological
treatments, such as cognitive therapy for depression, will be found to produce similar
effects, although those studies are not complete.

One critical issue that requires study is the extent to which there are important
environmental modulators of these brain systems early in life. These characteristics are
likely to be complex products of both genetic predisposition as well as environmental
components. Some data suggest, for example, that children with decreased activation
in the right prefrontal regions, and presumably with less well-developed skill at
processing certain types of negative cues, may be unable to experience the
consequences of punishment. In some environments, those children may be most likely
to display antisocial characteristics.